1. Field of the Invention
The present invention relates to a semiconductor laser diode array and more particularly to a semiconductor laser diode array which includes a plurality of semiconductor laser diodes separated by electrically conductive flexible or compliant spacers which minimizes tensile stress on the semiconductor laser diodes and in which connections are electrically connected by way of non-fluxed solders, hard and/or soft, such as eutectic solder, to eliminate various failure modes of known semiconductor diode laser arrays.
2. Description of the Prior Art
Semiconductor diode laser arrays are known in the art. Such semiconductor laser diode arrays are used in various applications including optical pumping of Nd: YAG slabs of lasing material used to form, for example, zig-zag optical amplifiers. Examples of such optical amplifiers are disclosed in commonly-owned U.S. Pat. Nos. 5,555,254; 5,646,773; 6,094,297 and 6,178,040. Such semiconductor diode laser arrays are used to optically excite the slabs to a relatively high-energy metastable state.
Such semiconductor diode laser arrays normally include a plurality of individual semiconductor laser diodes, commonly referred to as laser diode bars, which are electrically connected together and aligned so that the light path of each of the individual semiconductor laser diode bars is parallel. The laser diode bars are mounted to a thermally conductive substrate, such as a beryllium oxide BeO substrate. The substrate is used to conduct waste heat from the individual laser diode bars. The substrate, in turn, may be mounted to a microchannel, pin-fin, or labrynth type cooler for further cooling. Examples of such semiconductor diode laser arrays are disclosed in U.S. Pat. Nos. 5,040,187; 5,099,488; 5,305,304; 5,394,426; 5,438,580 and 5,835,518. Such semiconductor laser diode arrays are also disclosed in commonly-owned U.S. Pat. Nos. 5,748,654 and 6,208,677, hereby incorporated by reference.
Such laser diode bars are normally formed in a rectangular bar shape from various semiconductor materials such as GaAs, AlGa, As and InP semiconductor materials. Electrodes are normally formed on opposing longitudinal edges to allow such laser diode bars to be connected to an external source of electrical power. When such semiconductor laser diodes are configured in an array, the individual laser diode bars are connected in series. One electrode on each end of the array is connected to an external source of electrical power.
Various techniques are known for interconnecting the individual laser diode bars. For example, U.S. Pat. No. 5,040,187 discloses a substrate with a plurality of spaced apart parallel rectangular grooves. A continuous metallization pattern is formed from one end of the substrate to the other as well as in the walls of the grooves. The width of the grooves is selected to be slightly smaller than the width of the individual laser bars. The substrate is flexed to spread out the grooves apart to enable the laser diode bars to be inserted therein. When the substrate returns to a normal position, the laser diode bars are firmly compressed within the grooves to provide a secure electrical connection between the electrodes on the laser diode bars and the metallization laser on the substrate. Unfortunately, when packaged such an arrangement may cause tensile stress on the laser diode bars which can cause damage.
In order to avoid tensile stress on the laser diode bars, alternate methods for electrically interconnecting the laser diode bars have been developed. An example of such an interconnection method is illustrated in U.S. Pat. No. 5,305,344. In particular, the ""344 patent discloses a substrate with a plurality of spaced apart generally parallel grooves. A soft solder layer is disposed in each of the grooves. The laser diode bars are disposed in the grooves. Electrical connection between the laser diode bars is by reflow of the solder layer within the grooves. Unfortunately, the interconnection method disclosed in the ""344 patent results in various known failure modes, such as degradation of the laser diode bar, solder creep onto bar and contamination of the laser diode bars. Other known failure modes include alloying, melting, vaporization and arcing which can lead to a catastrophic destruction of the laser diode bars forming the array. Thus, there is a need for a semiconductor laser diode array that is fabricated in such a manner to eliminate known failure modes associated with fluxed soft soldering interconnection methods while at the same time minimizing stress caused by packaging to prevent damage to the laser diode arrays during assembly.
The present invention relates to a semiconductor laser diode array which includes a plurality of laser diode bars. Each of the laser diode bars is carried by a submount forming a subassembly. Each subassembly is separated by a flexible, compliant, or expansion-matched electrically conductive spacer. All connections within the array are by way of a non-fluxed solder, hard and/or soft, reflowed in a non-oxidizing atmosphere in a simple mechanical stack fixture to create nearly void-free solder joints with relatively high thermal integrity and electrical conductivity. Flexible, compliant or expansion-matched electrically conductive spacers are disposed between the subassemblies to substantially eliminate the stress on the laser diode bars while providing electrical conductivity between subassemblies. The subassemblies are carried by a thermally conductive dielectric substrate, such as a beryllium oxide, BeO, substrate, which, in turn, allows waste heat generated from the laser diode bars to be conducted to a cooling device, such a backplane cooler. In addition to eliminating known failure modes of semiconductor laser diode arrays in the solder interconnection as well as minimizing a stress on the diode arrays, the semiconductor laser diode array in accordance with the present invention is relatively simple and thus relatively inexpensive to fabricate and results in a more reliable semiconductor laser diode array with a useful life relatively longer than known devices.